Digital techniques for the transmission and reception of sound information, sometimes referred to as digital audio broadcasting (DAB), have progressed over the past few years and are anticipated, on a worldwide basis, to replace the present frequency modulation (FM) method of transmitting audio and other information. Digital audio broadcasting (DAB) is not only anticipated to replace FM modulation, but the fidelity of audio signals transmitted and received by DAB systems will be greatly enhanced, making DAB's acceptance welcomed worldwide.
One such DAB technique, the Eureka-147 digital audio broadcasting system, has been accepted around the world as an excellent technical solution for digital sound broadcasting to the mobile environment. The Eureka-147 DAB standard ETS300401 specifies a digital transmission technique for satellite, terrestrial, and cable distribution of sound and data in accordance with the Eureka-147 format.
The Eureka-147 standard ETS300401 specifies a Coded-Orthogonal Frequency Division Multiplex (COFDM) modulation technique, wherein 1.536 MHZ of bandwidth is occupied to combat frequency selective fading to mobile receivers. Such a bandwidth requires the transmission to include several program sources that are time multiplexed.
Referring to FIGS. 2A-2C, the format of broadcast information, in accordance with the Eureka-147 ETS300401 standard, is shown. The digital information 16 depicted in FIG. 2A is defined by data frames of information, such as frame 68, wherein each data frame 68 may be of length 24, 48 or 96 ms. Frame 68 defines a structure having a juxtaposition arrangement that includes a Synchronization Channel 70 which occurs first in time in frame 68, followed by a Fast Information Channel 72, which is followed by a Main Service Channel 74. The Main Service Channel 74 may contain 1, 2 or 4 Common Interleave Frames (CIF) as detailed in ETS300401.
The synchronization channel 70, shown in FIG. 2B, comprises synchronization symbols designated Null Symbol 76 and Time Frequency Phase Reference Symbol 78, which are used to synchronize a Eureka-147 receiver in time and frequency, and to obtain phase referencing. The Fast Information Channel 72 is used to define the Main Service Channel as well as convey several data services, and includes a number of data symbols illustrated in FIG. 2B as data symbols 72.sub.1, 72.sub.2, . . . 72.sub.j. The Fast Information Channel 72 may include either three or eight such data symbols.
Each Common Interleave Frame (hereinafter "CIF") contained within the Main Service Channel 74, such as CIF 80 shown in FIG. 2C, is made up of 55,296 bits which can be individually addressed in 64 bit allocations, or Capacity Units. CIF 80 thus comprises a maximum of 864 such Capacity Units 80.sub.1, 80.sub.2, 80.sub.3, . . . 80.sub.864. In accordance with the ETS300401 standard, the 864 Capacity Unit configuration of the CIF 80 is time division multiplexed by several information sources, which can represent compressed music (MPEG data), data in stream mode, or packet-data, wherein any of the information sources can range in data rate from 8-1728 kHz. Prior to time division multiplexing the various information sources onto the data frame 68, the sources are convolutionally encoded and pre-scrambled in time, or time-interleaved, over 16 CIFs (e.g. 384 ms). The resulting encoded information is transmitted for reception by an appropriately configured DAB receiver.
According to the ETS300401 standard, a CIF 80 may be multiplex reconfigured, meaning that the Capacity Units 80.sub.1 -80.sub.864 thereof may be dynamically redefined as they relate to the various information sources. Such dynamic redefinition of the Capacity Units may be manifested in either, or both, of two ways. First, any of the information sources may be moved dynamically to a different set of Capacity Units, wherein such moves may occur as frequently as once every six (6) seconds. A source occupying Capacity Units 50-79 may thus be moved, for example, to occupy Capacity Units 70-99. Secondly, an information source's data rate may increase or decrease at these same six second intervals. A source having a data rate requiring occupation of 30 Capacity Units may thus be dynamically decreased in data rate to require occupation of, for example, only 24 Capacity units, or may be increased in data rate to require occupation of, for example, 36 Capacity Units.
A DAB receiver must be capable of receiving the transmitted signals described hereinabove, synchronize with such signals in both time and carrier frequency, and decode the signal sources for replication of the original information. A Eureka-147 DAB receiver must therefore be able to time de-interleave 16 CIFS, de-puncture and viterbi decode the convolutional data, and store the decoded data for presentation to a data decoder or MPEG decoder at the various information sources' data rates.
In time de-interleaving (descrambling) such signals, known Eureka-147 based receiver systems typically utilize a single pointer arrangement to a memory unit which contains 16 CIFs of metric data at any time during the decoding process. Such brute force time de-interleaving techniques require a substantial amount of memory, thereby increasing the cost of the receiver 10. What is therefore needed is an efficient technique for time de-interleaving, de-puncture and viterbi decoding broadcast Eureka-147 based DAB information which minimizes memory required therefore.